1. Field of the Invention
The present invention relates to a thermosensitive color printing method and a thermosensitive color printer for use with a thermosensitive color recording medium. More particularly, the present invention relates to a thermosensitive color printer and an optical fixing method therefor, wherein the thermosensitive color recording medium is moved relative to a thermal head and an optical fixing device twice for one color frame, to do thermal recording and fixing of one color frame during the first relative movement, and a supplementary refixing of that color frame during the second relative movement.
2. Background Arts
The thermosensitive color recording medium consists of cyan (C), magenta (M) and yellow (Y) thermosensitive coloring layers that are formed on atop another on a support and develop the respective colors when heated. The obverse or the topmost thermosensitive coloring layer has the highest thermal sensitivity, whereas the bottommost thermosensitive coloring layer has the lowest thermal sensitivity. Because of the different thermosensitivities of the three coloring layers, three color frames are recorded sequentially from the obverse coloring layer by applying different ranges of heat energies for different colors. The heat energies are applied directly from a thermal head to the thermosensitive recording medium while it is moved relative to the thermal head.
After a color frame is recorded on the obverse coloring layer, e.g. the yellow thermosensitive coloring layer, coloring capacity of that coloring layer is dissolved by ultraviolet rays of a specific wavelength range. Thereby, the yellow thermosensitive coloring layer is optically fixed, and will not develop color even through higher heat energies are applied for recording a second color frame on the second obverse thermosensitive coloring layer, e.g. the magenta thermosensitive coloring layer. In the same way, the magenta color frame recorded on the magenta thermosensitive coloring layer is optically fixed by ultraviolet rays of another wavelength range. For the optical fixing, an ultraviolet lamp combined with a band-pass filter or two kinds of ultraviolet lamps are used.
Because the thermal sensitivity of the bottommost thermosensitive coloring layer, e.g. the cyan thermosensitive coloring layer, is so low that it would not usually develop color during the preservation, the cyan thermosensitive coloring layer is designed to maintain its coloring capacity. So any optical fixing process for the cyan thermosensitive coloring layer is not carried out.
Since the wavelength range of the ultraviolet rays for fixing the yellow thermosensitive coloring layer slightly overlap that of the ultraviolet rays for the magenta thermosensitive coloring layer, if the exposure amount to the yellow fixing ultraviolet rays is too large, it influences the coloring capacity of the magenta thermosensitive coloring layer. Therefore, the exposure amount to the yellow fixing ultraviolet rays is controlled to be constant by adjusting the radiant intensity of the ultraviolet lamp. On the other hand, since the yellow thermosensitive coloring layer is already fixed when to fix the magenta thermosensitive coloring layer, and also the ultraviolet rays do not affect the cyan thermosensitive coloring layer, the ultraviolet lamp is driven up to its maximum intensity to fix the magenta thermosensitive coloring layer without fail.
As well known in the art, the maximum radiant intensity of the ultraviolet lamp varies depending upon its tube temperature. That is, as shown in FIG. 14, when the ultraviolet lamp is driven by a drive pulse signal at duty factor of 100%, the radiant intensity increases with the tube temperature till it reaches a certain value. Thereafter, the intensity is maintained substantially constant, and above a certain higher limit TH of the tube temperature, the intensity begins to decrease with the tube temperature.
The radiant intensity of the ultraviolet lamp also depends on its running time. In the first stage of usage of the ultraviolet lamp, the radiant intensity increases with time from the start of driving the ultraviolet lamp, and after the intensity reaches a certain degree, it is maintained substantially unchanged with time, as shown by a chain-dotted line in FIG. 15. However, as the total running time increases, mercury is separated and deposited on inside of the tube of the ultraviolet lamp. Radiant intensity of the ultraviolet lamp that has the mercury deposited on the tube decreases with time after it reaches a certain degree, and thereafter increases with time again, as shown by a solid line in FIG. 15. The lowest value of the radiant intensity depends on the mercury deposit condition. It is to be noted that the curves shown in FIG. 15 is also obtained when the ultraviolet lamp is driven to the full by the drive pulse signal at 100% duty factor.
In order to prevent the yellow or the magenta thermosensitive coloring layer from being over- or under-exposed, or being fixed unevenly because of the variations in intensity of the ultraviolet lamp, U.S. Pat. No. 5,486,856 discloses an optical fixing method, wherein the thermosensitive recording medium is transported twice for one color under a specific ultraviolet lamp. Prior to the first transport, a maximum irradiance value of the ultraviolet lamp is measured as it is driven at pulse duty factor of 100%, and a value less than the maximum irradiance value is determined to be an irradiance set value. Then the thermosensitive recording medium is transported first at a first speed relative to the ultraviolet lamp while maintaining its irradiance at the set value. Since a thermal head disposed before the ultraviolet lamp records a color frame on a thermosensitive coloring layer during the first transport, the first speed is predetermined according to the thermosensitivity of that coloring layer to fix. Thereafter, for the supplementary refixing, the thermosensitive recording medium is transported for the second time relative to the ultraviolet lamp while maintaining its irradiance at the set value. The speed for the second relative movement is determined according to the irradiance set value, such that the total amount of exposure to the ultraviolet rays adds up to a predetermined proper value. Since the irradiance and the transporting speed are maintained constant during each transport, the entire area of the thermosensitive color recording medium is evenly fixed.
U.S. Pat. No. 5,892,530 discloses an optical fixing method, wherein a lowest irradiance value of a magenta fixing ultraviolet lamp is detected during a first transport of the thermosensitive recording medium through the magenta fixing ultraviolet lamp. Then, whether to refix the magenta thermosensitive coloring layer or not is determined depending upon the measured lowest irradiance value. If the exposure amount to the ultraviolet rays in the first or main fixing process is estimated to be insufficient, the thermosensitive color recording medium is transported for the second time under the magenta fixing ultraviolet lamp at a speed determined according to the lowest irradiance value. In this way, a supplemental amount of ultraviolet rays are projected onto the magenta thermosensitive coloring layer.
As described above, the radiant intensity of the ultraviolet lamp varies depending upon the tube temperature. Since the tube temperature at the start of the refixing is usually higher than before the main fixing, the maximum irradiance value can also be higher at the start of refixing than the value measured prior to the main fixing. Nevertheless, in the former prior art, the transport speed for the refixing is determined by the irradiance set value that is determined based on the maximum irradiance value measured prior to the main fixing. Therefore, the transport speed for the refixing can be too slow considering the capability of the ultraviolet lamp.
Also in the latter prior art, since the transport speed for the refixing is determined according to the lowest irradiance value so as to obtain a sufficient amount of supplemental exposure even if the irradiance of the ultraviolet lamp is maintained at the lowest value. Therefore, the transport speed for the refixing can be too slow considering the capability of the ultraviolet lamp. In other words, it may be possible to use a higher transport speed in combination with a higher irradiance value for the refixing. The higher transport speed results a shorter refixing times and thus a shorter total printing time.